Ionizing photon efficiency ξ_ion as a function of age (left panel) and Hα equivalent width (right panel) of different models and star formation histories. Solid (dashed) lines show models without (with) nebular continuum emission. In the right panel we also show for comparison predictions for metal-free (Pop III) populations extending up to 500 M_⊙ (blue dotted line). Dash-dotted lines show the “canonical” value of ξ_ion used by Robertson et al. (2013) (left panel), and the mean relation between ξ_ion and EW(Hα) derived by Tang et al. (2019) and Izotov et al. (2021) from observations.